1. Field of the Invention
The present invention relates to thermoplastic polymeric molding compositions; more particularly the present invention is related to miscible thermoplastic polymeric blend compositions comprising at least one polyamide, and at least one amorphous polyamide composition wherein the molding composition may be characterized as having good gas barrier properties, and/or good physical strength characteristics, even under conditions of high humidity.
2. Description of the Prior Art
As is known, there are exant a multitude of moldable thermoplastic compositions which comprise a polyamide. Polyamides are desirable engineering materials due to their excellent strength characteristics including impact strength and wear resistance which results from high crystallinity of such materials. They are readily processable and formable into a variety of articles and shapes, and which are readily available. However, polyamides are also known to the art to be particularly sensitive to moisture absorption, such as might be occasioned during use in humid conditions or wherein an article is contacted with water, as a consequence of which appreciable degradation of many desirable physical properties of the polyamide are known to result. Further, polyamides are known to exhibit poor vapor barrier properties to such gases as oxygen, and of course, water vapor.
In order to overcome these shortcomings, the prior art is replete with improved moldable thermoplastic compositions which suggest a broad range of additional constituents which may be used in conjunction with a polyamide in order to achieve selected improvements in such compositions.
U.S. Pat. No. 4,952,628 to Blatz describes thermoplastic blend materials comprising about 50-95 weight percent of an amorphous polyamide with about 5-50 weight percent of a copolymerized ethylene/vinyl alcohol polymer having an ethylene content of between 0-60%. The blend materials taught by Blatz feature physical properties such as reduced sensitivity to humidity, and improved barrier properties. The blends taught therein provide films and film layers within rigid packaging structures which feature good vapor barrier characteristics.
U.S. Pat. No. 4,983,719 to Fox et al. provides an amorphous polyamide composition which is a reaction product of a paraxylylene diamine, adipic acid and isophthalic acid; the polyamide composition features excellent oxygen barrier characteristics and finds particular use as a container layer in rigid food packaging structures.
U.S. Pat. No. 4,467,011 to Brooks et al. provides injection moldable compositions useful in forming laminates for films and rigid structures which include an amorphous polyamide and polyamide-imide copolymers. The compositions are particularly useful in forming coatings for glass fibers.
U.S. Pat. Nos. 4,788,248 and 4,788,249 to Maresca et al. provides thermoplastic resin blends which comprise a polyamide, a resin which may be a polycarbonate, polyester carbonate, or polyarylate, a compatibilizing copolymer of polyamide-polyester, and optionally a rubbery impact modifier. The compositions may include an amorphous polyamide which is derived from hexamethylene diamine and mixtures of terepthalic acid and isopthalic acid.
U.S. Pat. No. 4,014,967 to Kirsch et al. provides thermoplastic polyamide molding compositions which include at least one amorphous linear polyamide, and at least one segmented thermoplastic elastomeric copolyester; the elastomeric copolyester consist essentially of a large number of repeating intralinear long chain and short chain ester units linked head-to-tail via ester linkages wherein both the long chain and short chain ester linkages are of a particular structure.
U.S. Pat. No. 4,826,955 to Akkapeddi et al. provides an article of manufacture which comprises at least one barrier layer of an amorphous nylon copolymer.
PCT International Application WO 91/13113 to Exxon Chemical Patents Inc. provides oxygen barrier structures comprising polyoxamides or copoly(amide-oxamide)s which may be derived from a reaction between one or more diamines and oxalic acid, or derivative thereof. In Example 7 of that reference are discussed the copoly(amide-oxamide), nylon-MXD2/MXD6. The oxygen barrier layers find particular use in coextruded structures such a films.
Various other resins featuring reduced barrier characteristics may be found in "The Effect of Structure Upon the Oxygen Permeation Properties of Amorphous Polyamide" by TD Krizan, JC Coburn and PS Blatz, Polymer Preprints, 30, 9 (1989), which discusses blends of nylon 66, and a poly(hexamethylene isophthalamide/terepthalamide) resin. Further, in an article titled "Miscibility in blends of aliphatic polyamides and aromatic polyamide, nylon 3Me6T" by TS Ellis, published in POLYMER, 1988, Vol.29, (November), blends of nylon 6 and an amorphous aromatic polyamide, nylon 3Me6T, a condensation product of dimethyl terephthalate and 2,2,4-trimethyl-1,6-hexanediamine, and are discussed, and various nylons are discussed.
While these compositions provide useful thermoplastic compositions which find utility in the art, there nonetheless remains a real and continuing need for improved thermoplastic molding compositions which feature good physical characteristics, good vapor barrier properties and good processability.